Implants are artificial, that is, non-natural, structures that are implanted into a human or animal body, for example, in order to replace a joint entirely or partially in the form of an endoprosthesis. The affected joints can be, for example, a shoulder, hip, knee, or ankle joint.
A joint comprises two bone ends, that is, joint halves, which face one another in a movable manner with the respective joint or tribological faces. In the case of ball joints, such as the shoulder joint or hip joint, one joint part has a concave tribological face, for example the glenoid in the shoulder joint or the acetabulum in the hip joint, while the other tribological face of the tribological pairing, for example, the humerus head or upper-arm head in the case of the shoulder joint, or the femur head or thigh-bone head in the case of the hip joint, is embodied in a convex manner.
With regard to a joint replacement, there is a distinction between a total prosthesis and a hemi-prosthesis. A total prosthesis replaces the entire joint; that is, each of the two joint halves is replaced by an implant, wherein the implant is anchored in the remaining bone. The tribological pairing between the convex and concave replacement-joint parts is mediated exclusively by artificial materials.
A hemi-prosthesis replaces only one joint half, generally the convex tribological partner. By comparison with a total prosthesis, the intervention is less serious and the stress on the patient is reduced. A hemi-prosthesis can be selected, for example, if the cartilage of the natural, concave tribological face of the glenoid, acetabulum, or tibial plateau in the knee, etc. is healthy and intact. For example, in the case of the hip joint, with a damaged femur head or fractured neck of the femur, both parts of the femur bone are replaced, but, with regard to the pelvis side, the acetabulum is retained.
Replacement joint parts must be sufficiently strong to transfer the forces acting on the joints in the long-term. Accordingly, implants in this region are often manufactured from metal or ceramic. However, in the case of hemi-prostheses there is the problem that strong prosthesis materials are not good frictional partners for the natural cartilage of the other joint half. Accordingly, it is known that, for example, steel or cobalt alloys cause damage which can go as far as destroying the cartilage.
Various materials which can be used as tribological partners for the cartilage instead of metal or ceramic have been suggested. So far, results relating to their actual suitability are available only for a few of these materials. However, the materials can be categorized by comparison with metal or ceramic roughly into rigid or hard, on one hand, or less rigid or less hard materials. Examples of hard materials include, for example, oxide-ceramic coated metals, such as oxidized zirconium, oxidized ceramic, pyrolytic carbon (pyrocarbon).
Examples of materials tending to be less rigid include various synthetic materials. At least some of these materials are not strong enough to be used as base materials for the manufacture of implants such as replacement joint parts. A corresponding implant must therefore be built up in two parts, that is, it must comprise a strong material with a coating made from a cartilage-sparing material. However, how a rigid substrate material can be joined permanently and inseparably with a less rigid coating material with regard to the forces acting in a joint remains problematic.
It is known from EP 2 086 471 B1 that a pyrocarbon shell can be fixed onto a metallic carrier. In this context, a polyethylene intermediate layer is provided. The pyrocarbon shell is embodied with a bottom edge directed inwards, which engages in a corresponding peripheral recess of the polyethylene intermediate layer, thereby providing a compression fit. The polyethylene casing and the metallic carrier each provides a corresponding peripheral groove in which an annular holding ring is retained. This arrangement can be suitable for the attachment of rigid materials such as pyrocarbon but cannot be transferred to other, less rigid materials.